Set analysis and field selections behaviour

[#1]

I have three fairly simple expresssions

SUM({$<SECTOR-={"ITRAXX*","CROSSOVER*"},DESK={"CDS CORPORATES"}>}[TRADE COUNT])

SUM({$<DESK={"CDS EMERGING MKTS"}>}[TRADE COUNT])

SUM({$<DESK={"BANK CAPITAL - GSL"}>}[TRADE COUNT])

they are displayed as 3 lines on a line chart

I also have the list box fields of DESK and SECTOR on the page.

My problem is that when I select the 'BANK CAPITAL - GSL' value in the DESK list box, the other two lines remain on the chart, rather than disappearing as i was expecting. Why is this? How can I get round the issue?

Incentally the same thing happens if I select 'CDS EMERGING MKTS' in the DESK list box

[#2]

GetFieldSelections(DESK) = 'CDS CORPORATES' or GetPossibleCount(DESK) = count(DISTINCT {1} DESK)